1. Technical Field of the Invention
The present invention relates to image forming apparatus such as copying machines, printers, and facsimile machines, for, when sheets with images formed thereon are ejected to a sheet eject section, sorting the sheets to be ejected by displacing the sheets in a direction orthogonal to a sheet transport direction.
2. Description of Related Art
Because of increasing unitization of image forming apparatus, peripheral devices for performing specific functions that a user desires are selectively combined to be installed in a body of the image forming apparatus. However some of the functions do not require installation of additional peripheral device(s), and some users do not desire such a function as to involve the installation of additional peripheral device(s). If a user does not desire such a post-processing function as of sorting or stacking sheets to which image forming processing is performed, it is not necessary to install a post-processing device for performing the sorting or stacking processing as a peripheral device in the image forming apparatus. In this case, all the sheets with images formed thereon are ejected to and stacked on a single sheet eject tray.
If an image forming apparatus is not provided with the post-processing device as a peripheral device and thus all the sheets with images formed thereon are ejected to and stacked on a single sheet eject tray, when image forming processing with regard to a plurality of image forming jobs directed by a plurality of users is performed in succession, sheets with images formed thereon in the corresponding processing to the respective jobs are stacked one on another on the single sheet eject tray. Thus it is difficult for the users to distinguish among the sheets, and the users have trouble in taking out of the sheet eject tray the sheets corresponding to the jobs they instructed. In particular if the sheets are ejected face down, namely, with image-formed sides thereof down, the users must turn over the sheets on the sheet eject tray, sheet by sheet, to find their corresponding sheets. This problem also occurs in a single user when image forming processing is performed to plural copies of sheets with each copy containing a plurality of sheets.
To solve the above problem, one example of conventional image forming apparatus is provided with a mechanism for displacing sheets selectively to a plurality of positions in a direction orthogonal to a sheet transport direction and ejecting the sheets onto a sheet eject tray. In an image forming apparatus without a post-processing device as a peripheral device, the arrangement allows sheets with images formed thereon in respective image forming processing performed in succession to be sorted on a single sheet eject tray without difficulty. Consequently, each user can easily take out of the sheet eject tray the sheets corresponding to the image forming processing the user directs.
For displacing a sheet eject position selectively in a direction orthogonal to a sheet transport direction as described above, there is a method in which the sheet eject tray is displaced in the direction orthogonal to the sheet transport direction. There is also another method in which a sheet transport mechanism for guiding a sheet to the sheet eject tray is displaced in the direction orthogonal to the sheet transport direction. Since a complicated device is required for displacing a sheet halfway on a sheet transport path provided within an image forming apparatus, in most of the image forming apparatus according to the latter method a sheet eject mechanism supporting sheet eject rollers rotatably at a sheet eject slot is arranged to be displaced in the direction orthogonal to the sheet transport direction.
In this case, the sheet eject mechanism is arranged in the vicinity of the sheet eject slot to be movable between an initial position and a sorting position in the direction orthogonal to the sheet transport direction. In displacing a sheet to be ejected to the sheet eject tray from the initial position to the sorting position in the direction orthogonal to the sheet transport direction, the sheet eject mechanism moves from the initial position to the sorting position after the front end of the sheet passes through sheet eject rollers; and then the mechanism returns from the sorting position to the initial position after the rear end of the sheet passes through the sheet eject rollers.
Arranged opposite a sheet-stacking side of the sheet eject tray, the sheet eject mechanism is in most cases provided with a contact-type sensor for detecting height of sheets stacked on the sheet eject tray. The sensor has an actuator swingably mounted thereon which extends downward and abuts on the uppermost sheet of the sheet stack on the sheet eject tray. According to a swing angle of the actuator, the sensor detects the height of sheets stacked on the sheet eject tray.
Since a sheet whose rear end passes through the sheet eject rollers is affected by air resistance while it is falling toward the sheet eject tray, the sheet takes a certain time to fall from the sheet eject mechanism to a level where the sheet does not get contact with the actuator of the contact-type sensor. In general, the sheet-stacking side of the sheet eject tray slopes down to a body of the image forming apparatus so that the single contact-type sensor arranged at a part of the body above the sheet eject tray detects the height of the sheet stack with various sizes of sheets.
Therefore a sheet ejected by the sheet eject rollers, after falling onto the uppermost sheet of the sheet stack on the sheet eject tray, moves in a direction opposite to a sheet eject direction along the slope of the sheet-stacking side, to come to a stop with rear end thereof abutting on an end of the sheet eject tray on the side of the body of the image forming apparatus.
As the height of the sheet stack on the sheet eject tray reaches a full level, the actuator of the contact-type sensor is in constant contact with the uppermost sheet of the sheet stack on the sheet eject tray. When the sheet eject mechanism starts to return from the sorting position to the initial position while an ejected sheet is moving in the direction opposite to the sheet eject direction, the unstable sheet on the move is displaced in the direction orthogonal to the sheet eject direction by external force from the actuator, causing the sheet to be placed in a disorderly way on the sheet eject tray. Accordingly, it is necessary to set the sheet eject mechanism returning to the initial position in a sufficiently delay time after the rear end of the sheet passes through the sheet eject rollers.
After the passage, it is necessary for the sheet eject mechanism to return to the initial position before the front end of a following sheet reaches the sheet eject rollers. It being considered that a transport interval between sheets tends to be shorter with speeding up of image-forming speed, it is necessary to set the sheet eject mechanism returning at the quickest timing possible after the passage through the sheet eject rollers of the rear end of a preceding sheet such that the front end of the following sheet is appropriately held between the sheet eject rollers.
The foregoing necessity causes the following problem. The sheet eject mechanism is set returning before a sheet whose rear end passes through the sheet eject rollers falls with a sufficient distance from the sheet eject mechanism. Or the sheet gets contact with the actuator of the contact-type sensor while moving in the direction opposite to the sheet eject direction after ejected by the sheet eject rollers. Thus, ejected sheets cannot be stacked neatly at a predetermined position on the sheet eject tray, with the result that the sheets cannot be sorted properly.